1. Field of the Invention
The present invention relates to a limiting circuit for limiting a voltage or current to be input with a predetermined value, and an electric motor driving device for efficiently driving an electric motor by using the limiting circuit.
2. Description of the Related Art
Conventionally, a spindle motor has been used for driving a disc such as a CD (Compact Disk) or a DVD (Digital Video Disk).
FIG. 8 is a diagram showing the structure of a driving device for a three-phase brushless electric motor M to be used for the conventional driving operation, which has been described in JP 2002-84772. In the conventional example, the electric motor M is constituted by a permanent magnet rotor, and a stator in which armature coils having three phases of a U phase, a V phase and a W phase are provided on a circumference and a rotor position detector for each phase is provided in the position of the armature coil having its respective phase. A rotor position detector 11 for each phase is representatively shown on the outside of the electric motor M.
In FIG. 8, a transistor switch for each phase is constituted by P-type MOS transistors QUH, QVH and QWH on a positive electrode side and N-type MOS transistors QUL, QVL and QWL on a negative electrode side, and they are ON/OFF controlled in accordance with a gate control signal respectively.
The rotor position detector 11 is constituted by a Hall device, for example, and outputs six types of sine wave signals including output signals on positive and negative electrodes in U, V and W phases, where a phase difference in the output signal in each phase is 120 degrees (=360 degrees/3).
A position detector/phase shifting circuit 14 takes a difference between the output signals on the positive and negative electrodes for each phase in signals HU, HV and HW output from the rotor position detector 11, and removes an in-phase noise component superposed on a signal line, obtains mutual difference signals of the output signals HU, HV and HW and outputs the phase-shifting signals HU1, HV1 and HW1, having a phase difference Δθ of 30 degrees, for example.
The phase-shifting signals HU1, HV1 and HW1 are mainly formed for the following reason. More specifically, a lag corresponding to a time constant is generated by the inductance component of the armature of the electric motor M before a voltage is applied to the armature and a current actually flows out upon receipt of a signal sent from the rotor position detector 11, and the commutation time of the current flowing to the armature is later than a normal commutation timing so that an electric motor driving efficiency is deteriorated or a torque unevenness is increased. Therefore, such a situation shall be prevented from being caused.
An oscillator 13A includes a triangular wave generating circuit constituted by an operational amplifier, a constant current source and a capacitor, and generates a triangular-wave high-frequency reference signal OSC having an audio frequency band (16 kHz) or more, for example, and outputs the same signal OSC to a comparator 16A.
The comparator 16A receives the phase-shifting signals HU1, HV1 and HW1 and the triangular-wave oscillating signal OSC sent from the oscillator 13A and compares them with each other, and outputs PWM signals UPWM, VPWM and WPWM from a difference between both of the signals.
Pre-driving circuits 17AU, 17AV and 17AW for each phase receive the PWM signals UPWM, VPWM and WPWM sent from the comparator 16A every phase. Gate control signals VUGH to VWGL are formed by the PWM signals UPWM to WPWM, and are supplied to P-type MOS transistors QUH, QVH and QWH on the positive electrode side and N-type MOS transistors QUL, QVL and QWL on the negative electrode side.
A torque command circuit 12 serves to output a control command in such a manner that the rotating speed of the electric motor M has a predetermined value, and to compare a set value Vs of the rotating speed with a measured value Vdet of an actual rotating speed and to control the amplitudes of the displacement signals HU1, HV1 and HW1 corresponding to a deviation thereof.
In the structure described above, the measured value Vdet which is proportional to the actual rotating speed of the electric motor M is detected. For example, when the speed of the electric motor M is higher than the predetermined set value Vs, a control signal corresponding to a deviation thereof is output to the position detector/phase-shifting circuit 14 to reduce the amplitudes of the displacement signals HU1, HV1 and HW1.
By the reduction in the amplitudes of the displacement signals HU1, HV1 and HW1, the pulse width of an ON/OFF duty in the PWM signals UPWM, VPWM and WPWM sent from the comparator 16A is shortened, a current flowing to the electric motor M is decreased through transistor switches QUH to QWL for the U, V and W phases to decelerate the electric motor M. Also in the case in which the rotating speed is low, similarly, the current flowing to the electric motor M is increased to accelerate the electric motor M. Thus, the speed of the electric motor M is controlled.
However, when the three-phase brushless electric motor M is to be driven at the highest rotating speed, for example, the set value Vs (that is, a reference voltage) is increased in order to obtain a large torque. When the rotating speed of the electric motor M is increased, the rotating electromotive voltage of the electric motor M is raised so that a driving current flowing to the electric motor M is decreased. Consequently, such a control as to raise a voltage applied to the electric motor Miscarried out. When the applied voltage is excessively raised, a driving current waveform is distorted by the influence of the rotating electromotive voltage and the applied voltage so that a driving efficiency is deteriorated. Moreover, the electric motor M cannot be driven with a sine wave-shaped current. Consequently, there is also a problem in that a noise is increased with the driving operation of the electric motor M.